Add mimimum width of the buttonPanel layout

Some button texts are not shown correctly (in one row) in
some languages, e.g. Español.

This is fixed by adding a minimum width to the buttonPanel
for the stk_msg_dialog layout.

Change-Id: I1c028011d52af92bfaccc700f3fab76d86f9c786
Automatic translation import

Change-Id: I2935f0bbf79c59a8813def49b2b6577b2fb8e5da