Fix snippets keybindings not blocking handled events

Closes #1360.

Fix signature of the snippets keybindings callback, properly blocking
further propagation of handled events thus avoiding possibly activating
another action (like a builtin Scintilla keybinding).

Fixes #1354.

api: rework GeanyProxyProbeResults compat symbols as defines

This is a follow-up of #485.

Constants with different namespace in the same enum confuse g-ir-scanner.

Fix the current scope shown in the statusbar

Since the Scintilla C++ lexer started to fold on `()` [1], the code
looking up the current scope is confused whenever the function
signature spans multiple lines.  Fix this by skipping fold levels that
correspond to parentheses.

Fixes #1279.

[1] https://sourceforge.net/p/scintilla/feature-requests/1138/
    imported in 24f91981

Fix some various issues with accessibility code in Scintilla.

We cannot compute the length in characters after the text has been
deleted, so we need to compute it in BEFOREDELETE.  However, we need to
emit the signal once the buffer has actually changed, so we need to
cache the value in-between those events.

This doesn't support nested templates or escaped `es, but at least
handles the most obvious cases for now.

Part of #934.

Closes #1339

It actually introduces a few new things worth mentioning.

Say hello to Geany 1.30 "Calron"!

Windows: Improve executing external commands on non-ASCII paths,
effectively fixing most known issues.

Use pre-compiled 'grep' binary from MSYS2 on Windows

This restores the previous behavior as it has been on non-Windows
systems before.
Post-1.29 we will merge #1300 which implements run helper script support
for non-Windows systems more sophisticated.

utils_get_utf8_from_locale() is actually a no-op on Windows, so use the
GLib conversion directly.  We really mean locale here, not "filename
encoding".

Apparently using arguments instead of putting paths directly in the
script is enough for it to work on Windows, so use a simple script
instead of a program, so it's both shorter and easier to tune.