add search shortcuts

Hello,

xfmpc has one of the best search tool for mpd databases in my oppinion. But switching between playlist, database and entry field can be a little bit of a pain, requiring too much mouse clicks. Therefore I added a few direct shortcuts. Possibly you find this useful, too?

added keyboard shortcuts

• alt+s -> jump into [S]earch/filter entry field
• alt+f -> switch to playlist and focus [F]ilter entry field
• alt+d -> switch to [D]atabase and focus search entry field and updated shortcuts-dialog accordingly

Note: Setting focus to the entry field is realized by grab_focus onto the playlist/database combobox followed by 2x move_focus forward. This works well but is not perfect. However, trials to directly grab_focus onto the entry field failed, and it seems to require much more changes.

Best regards

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Closes: #3 (closed)