When editor_create_widget() is used on an editor with already existing
widget, don't reset the internal indentation settings of that editor.

Closes #189

3e089e4c accidentally switched from
`g_get_user_config_dir()` to `g_get_user_data_dir()` as the location
for the user's configuration directory.

Force disk check before save to prevent accidental overwriting

Add project prefs for line breaking column & multiline comment completion

Remove unused tag manager files

Before we used g_get_user_config_dir() but GLib changed the returned
location in newer versions, so use the Windows API directly
to get the old location, at least for now.
Also add utils_get_user_config_dir() wrapper.

Code is based almost completely on a patch from Matthew.

Fix typo in README

Useful e.g. with Save All when the modified file is not the current tab.

Don't use `g_regex_get_compile_flags()` that is new in 2.26, but we
only depend on 2.20.

Closes #340.

Project line wrapping pref

Fix adding stash group and applying defaults.
Don't let Properties dialog override per-document line wrapping state.
Use "editor" for key group name.
Remove code already handled by stash group.

Notify user when trying to save read-only document

Confirm infobar reload when document has modifications

New name is clearer against document_reload_prompt.
Add deprecated alias.