Improve editing mode button and visibillity of current mode status

[deinhofer]

Currently, it's not clear if you are in editing or in viewing mode.

View-Mode: Editing-Mode:

Improvement suggestion

Button ```Editing on/off```` indicators

According to Modes in User Interfaces: When They Help and When They Hurt Users there are several possibillities how to emphasize the current mode:

1. Change colour (of button or whole ui background?)
2. Change cursor
3. Tooltips that show what will happen, when clicking the button.

"...Redundancy in indicators is a wise idea: we strongly recommend at least two visual indicators (such as the combination of highlighting on mode-selector controls combined with a cursor change)..."

Open Edit action in cell

To simplify the opening of the edit action, each cell could get an edit icon overlayed to it. The context menu should still exist, but only for other operations...