Xfce4-panel 4.14 fixes

[SibrenVasse]

Three fixups for xfce 4.14: Original PR: (https://github.com/jnsh/arc-theme/pull/11, https://github.com/jnsh/arc-theme/pull/10)

• The font-weight of the taskbar changed: Old (xfce4.12) New (xfce4.14) New (xfce4.14) with PR

• Ugly border around the systray with 'show frame' enabled: Old (xfce4.12) New (xfce4.14) New (xfce4.14) with PR

• Since xfce 4.14 the 'Window Buttons' panel visuals are 'broken' if the 'show flat buttons' option is not enabled.

This setting used to have no effect on the visuals on 4.12.

Old situation (xfce4-panel 4.12 - flat enabled): Old situation (xfce4-panel 4.12 - flat disabled):

Broken (xfce4-panel 4.14 - flat disabled):

New (xfce4-panel 4.14 with this PR - flat enabled): New (xfce4-panel 4.14 with this PR - flat disabled):

[fossfreedom]

merci beaucoup!