Rename the "Dark Frame" theme to "Default Flat"

Change-Id: I1d2dc8af94466de29fc1429c82dbbd37bcf7310b
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

share/qtcreator/themes/dark.creatortheme

 [General]
-ThemeName=dark
+ThemeName=Dark
 PreferredStyles=Fusion
 DefaultTextEditorColorScheme=dark.xml
 

share/qtcreator/themes/default.creatortheme

 [General]
-ThemeName=default
+ThemeName=Default
 PreferredStyles=
 
 [Palette]

share/qtcreator/themes/darkframe.creatortheme → share/qtcreator/themes/defaultflat.creatortheme

 [General]
-ThemeName=Dark Frame
+ThemeName=Default Flat
 PreferredStyles=
 
 [Palette]