* Mention GTK 3 for the first time in HACKING and say how to select.

Fixed misspellings

This fixes minor problem which affected C++ plugins since
3e649ddd where they would have to add
a the cast themselves.

Returning large aggregates as values is a potential performance problem, but preventing all aggregate returns is inappropriate because:

1. returning small aggregates that are mostly handled as complete objects as return values often makes code clearer.

2. Modern (well not so modern actually) ABIs allow for small aggregates to be returned in registers, so forcing all returns to memory is actually likely to be slower.

3. if code review does not notice a large aggregate return and nobody notices a performance issue then it doesn't matter

This was introduced in 1f71ccd4 because the VTE widget's existing
button-press handler didn't return `TRUE` and so the event propogated
up to the notebook showing both of their context menus.

For some reason the TravisCI build has `-Waggregate-return` enabled as
an error even though it's completely valid code. Also constify the
input `color_name` argument to `load_color` function.

* Improve GTK+3 theme loading

- Merge geany-3.0.css into geany.css
- Load geany-3.20.css to override if runtime version dictates
- Load geany.css from app->configdir if it exists.

* Add initial documentation for GTK+ CSS theming

* Add geany.css file to the `Tools->Configuration Files` menu

Also mark such config files as changed if they don't already exist to
force the user to save or discard any newly created config files.

* Allow customizing message window using GTK+ themes

* Improve GTK+3 CSS docs a bit

State which classes are meant to be overridden by themes.

* Go back to conditionally loading geany-3.0.css

It can't be merged into geany.css since GTK+ CSS parser doesn't ignore
invalid properties when prefixed with vendor extension (ie. -Gtk).

* Don't mark unedited config files as changed

They probably should be marked as changed since they don't exist on
disk yet, but for the time being leave it how it was.

* Fix misuse of CSS ID vs class in manual

* Minor colour value tweaks

Based on feedback from @b4n

Show status message on attempt to execute empty context action.

If a user selects "context action" from the context menu then now the status message
'No context action set.' will be shown. Closes #1641.

Simply zero-initializes the variable.

Requires -O3 optimization level to trigger.

Requires -O3 optimization level to trigger the warning.

Discussed in and fixes #1683

To document when switch cases fall through intentionally, hiding the
warning by GCC at the same time.

Moved the break statement outside of the if conditional in keybindings.c
as I believe this was an actual bug found by GCC's warnings.

Where an enumerator was treated as signed.

These were introduced in #1470 and a fix was also provided in #1554.

TODO: make sure these are actually meant to fall-through.

As per the documentation[0], GCC will silence these warnings when
the comments are formatted correctly.

[0]: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

This is basically a false-positive by GCC since static variables are
unambiguously zeroed-out per the language standard, but the workaround
is simple enough.

Add a note for applying the indent settings in the project preferences

Fix the symbols tree hierarchy when several tags have the same name