isValid -> supportsEditor

Reviewed-by: Roberto Raggi

javascripteditor: compile fix Linux, compile fix namespaces

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.

Introduce setCodeFoldingSupported to the BaseTextEditor, and only
enabled it for the C++ and QtScript editors. This removes the useless
margin meant for code folding markers from the plain text editor and
.pro file editor.

Task:     -
RevBy:    Thorbjorn
AutoTest: -

in place
RevBy:   con

Reviewed-by: Friedemann Kleint

RevBy: Roberto

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.

Details:  - Naming convention