[kde-guidelines] Minimal app size

[Heiko Tietze]

During the Akademy we spotted the issue that the HIG is unclear about minimum 
size. And recently this issue came up again in the forums [1]. 

"Make sure your application works with at least 1024x768px, which is the 
minimum for KDE SC." (Placement)

Luebking recommends to make to HIG somewhat more suggestive, just to 
illustrate the problem:

* KDE seeks to support XGA (1024x768px).
** Keep in mind that not everyone is provided with a high resolution display.
** Avoid to have a large number of controls visible at once, which in turn 
requires a huge minimal size.
** Keep in mind that the available screen area typically also will be shrunk 
by panels and the window titlebar. Also, user's font might be bigger than 
yours (e.g. for accessibility reason).
** You therefore should ideally preserve ~10% to catch those variables and try 
to not exceed 920x690px.

Furthermore we should consider to remove "Modal dialogs must not be resized." 
from the Placement page.

Any objection to these changes?

[1] 
https://forum.kde.org/viewtopic.php?f=285&t=122989&sid=c06ebaf5ce69bebd0694cc0c6b7b01b9&start=15#p320708