tdf#96316: Decouple view-only/editable modes

- Move text selection and graphic selection tasks into functions
- Merge GDK_BUTTON_PRESS, GDK_BUTTON_RELEASE conditional code
- Do not change to 'move' cursor in view-only mode
- Ignore LOK_POST_COMMAND, LOK_SET_GRAPHIC_SELECTION in view-only

As a consequence this commit also allows dragging handles during text
selection in view-only mode which was earlier not possible.

Change-Id: Iffb668d5447dd646a1e40237dee8d8d3fa3314b6
Reviewed-on: https://gerrit.libreoffice.org/20487Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>