Add settings page allowing user to change framebuffer plugin

Added a new page to config dialog: "Screen capture", allowing to choose a framebuffer plugin.
Added a brief description about xcb and qt plugins.
Changed an icon for first tab "Network" to more appropriate one.
Code is prepared to expect "x11" as preferredFramebufferPlugin and automatically change this setting to "xcb".
Do not hardcode possible list of framebuffer plugins in combobox, but instead find actual available plugins at runtime.
If preferred plugin has changed in settings, inform user that krfb needs to be restarted to apply changes.

Differential Revision: https://phabricator.kde.org/D6319