Respect GTK2_RC_FILES environmental variable

Summary:
If the above variable is set and it does not contain multiple paths,
write GTK2 config into the designated config file, instead of the
default one.

BUG: 415770
FIXED-IN: 5.18.0

Test Plan:
Check if the custom file provided by env variable is modified, when changing any settings, that are synchronized by kded module (icon theme, cursor theme etc)
Simple test:

1. mv ~/.gtkrc-2.0 ~/.gtkrc-2.0.bak
2. export GTK2_RC_FILES=~/.gtkrc-2.0.bak
3. Restart kded5 with the above env variable
4. Change e.g. icon theme and see, that .gtkrc-2.0.bak is modified, but .gtkrc-2.0

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26375