Icon in default view mode works fine - but cannot be changed

[Lefteous]

Preconditions:
- TC 9, beta 4, x64
- There is an icon defined for default view mode
- There are a few tabs opened in default view mode

1. Open TC. The tabs with default view mode display the defined icons properly.
2. Now we wanna change the icon and open the settings dialog, page 'view mode'
3. The defined icon is displayed in the settings of default view - but it CANNOT be changed as the browse for icon button is disabled.

Expected: Enable the browse for icon button

[Lefteous]

10.07.16 Added: Allow to define a tab icon for the default view mode (32/64)

This now works fine in beta 5 - thank you!