Write Fonts settings to LXQt config

[sledgehammer999]

Partially implements #1022

With this, functionality is not lost or changed.
The fontsconfig is still written, however all related values are now saved/loaded to lxqt.conf first.

There's one caveat because I don't your policy on this. If this gets merged as-is the user's old settings will be lost. The user will need to customize them again.
Do you want a migration code that will load the customized settings into the lxqt.conf from fonts.conf if they are missing? It will makes things much more complicated though...

If this gets approve the future PRs will be in these stages:

• stage 2: Write the Fonts settings into the appropriate values of xsettings/gsettings/settings.ini
• stage 3: Yank out the code for xsettings and migrate it to lxqt-session. Make the xsettings deamon start at session start and continue running for the whole session. Reload it when changes are done to the settings.
• stage 4: Investigate what needs to be done on the lxqt-qtplugin side.

[sledgehammer999]

Don't get confused by the diff. The actual changes are minimal. I moved code around and that resulted in a big diff.