ComboBox: respect style padding for the editor

Get rid of the hard coded 8px left margin and use whatever value
the style supplies (and is already used for the label). This allows
the Android style to provide a sensible value for High DPI screens.

The new value for the left margin is 6px in Base and Desktop styles.
It looks slightly better, but still not pixel perfect. Notice that
the text label of editable and non-editable ComboBoxes are not
perfectly aligned in native Windows and OS X ComboBoxes either.

Change-Id: I9a66c3a26fcd9ef1653b355e831277a7b6cbe72f
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Fix TextHandle min/max position handling

When not explicitly specified, the maximum is the length of the editor
content, not minus one (cursor can be moved "past" the last character).

Furthermore, due to binding evaluation orders, the minimum/maximum
properties might be temporarily pointing outside the range so we must
validate them to avoid clients (TextField/TextArea) having to do that.

Change-Id: I1faddd1e7ac4f12d7a802484ceb8eea500ad8bfc
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>