[appindicator] Force update indicator menu on clipboard changes

[BLaDZer]

If copied text already in menu list then indicator menu wasn't updated and display wrong clipboard status. For example:

• we/or another program copy the same text already copied before
• or just when both "Use Copy" and "Use Primary" options are enabled)

Step to reproduce:

1. need configure project with "--enable-appindicator" like in Ubuntu (at least)

   ./configure --with-gtk3 --enable-appindicator \<compile...>

2. in settings enable both "Use Copy" and "Use Primary" options
3. now select text with mouse(first clipboard) and then press Ctrl+C (second clipboard)
4. check indicator menu in tray (text should be in italics but not bold)

This bug is not affected by program compiled without "--enable-appindicator" option

I think this could be related to #108