Reviewed-by: dt

Before we cached based on the main qml url - This way we skip setting up
the specific widgets each time we change selection

This patch allows setting up property panes before a selection is made

Only update geometry based properties in  transformChanged()

 * Initialize it to the toolchain used, not the preferred one

Reviewed-by: Robert Loehning

* Add SemanticHighlighter to QmlJSTextEditor to update the semantic info
  in a background thread.
* Add QmlJS::Check to run semantic checks on qml and js documents.
* Add a check for incorrect property names.
* Fix hoverhandler to show tool tips from extra selections over help
  tooltips.

Display correct warning if git is not installed.
Do not display it as error (popping up the log pane) from
IVersionControl.  Introduce separate caching flag to avoid
repeated execution if git is not installed.

Backspacing across start of line was broken, and it always removed all
automatically indented whitespace.

Returning a value is not yet supported.

When closing editor splits while the same document was visible in
two editors, it could happen that the state of the closed editor
survived (as opposed to sate of the editor which was still visible).
This had the negative effect, that when visiting the file again,
the cursor would jump to the obsolete position within the file.

Task-number: QTCREATORBUG-188