Tweak column widths in tree view of file open/save dialogs

Summary:
Adjust column sizing options so that "last" column is no longer the largest.
First column (folder/file name & icon) will now occupy the majority of the space but does not compress other columns.

BUG: 354388
BUG: 338502
BUG: 196508
BUG: 177743
BUG: 96638
FIXED-IN: 5.46

Test Plan:

• Compile KIO with patch
• Recompile an application (i.e. Kate), ensuring it will include the patched version of KIO
• File -> Open (and/or File -> Save); select Detailed Tree View option
• Check that Name column is the widest and occupies most of the space

Reviewers: ngraham, dfaure, Frameworks, broulik

Reviewed By: ngraham, broulik

Subscribers: Dolphin, cfeck, fabiank, broulik

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11993