Add flag indication user can change Read only mode for view

git-svn-id: http://tortoisesvn.googlecode.com/svn/trunk/src/TortoiseMerge@24075 4cfb86a4-2c7a-11df-bd14-c1e331e6ceac
* Add the "Enable Edit mode" button to the ribbon and the File menu
* Set the button as enabled if it's possible to change the readonly state
* Set the button as checked if the current view is editable
* remove state indication from the view header

git-svn-id: http://tortoisesvn.googlecode.com/svn/trunk/src/TortoiseMerge@24076 4cfb86a4-2c7a-11df-bd14-c1e331e6ceac