The actions are already lazily instantiated in
TextEditorActionHandler::setupActions(), so there should be no need to
call initializeActions() in all plugins on startup. Only the main text
editor plugin will still initialize its actions, to make sure the menu
items are there.

This stuff was all over the place, a relic from before the editor
settings were unified. Now the initialization is only done in
TextEditorSettings.

Fixes the .pro file editor and the generic project files editor, which
before only used the font settings.

Details:
This patch makes use of system icons as a base
and draws overlay on top. This should blend a bit better
into the native icon theme.

Many singletons have their own instance() method, in which case there is
no need to use ICore::instance() to get access to them.

replace ExtensionSystem::PluginManager::instance()->getObject<Core::ICore>() by Core::ICore::instance()

forward declaration is sufficient.

Now reuses the colors set for Type and Keyword for the variables and
functions respectively. Having the categories Variable and Function in
the fonts and color settings was confusing.