mpris: reduce command latency in event loop

[iamcvr]

Summary

• Remove the fixed 500ms sleep from `MPRISRemote::MPRISLoop()` in `src/plugins/mpris/mpris.cpp`.
• Drain pending DBus messages each loop iteration before waiting.
• Reduce control latency for MPRIS actions (e.g. `PlayPause`) from noticeable delay to near-instant in local testing.

Context

This targets reported lag when controlling musikcube over MPRIS/playerctl (see issue #551).

Testing

• Built locally with MPRIS plugin enabled.
• Reproduced latency before patch and verified significantly improved response after patch using `playerctl` and `busctl`.

Baseline validation (control path overhead)

Command:
bash -lc 'set -x; date +%s.%3N; time qs -c noctalia-shell ipc call media playPause; date +%s.%3N; playerctl -l; playerctl -p musikcube.instance791874 status'

Observed:

• qs ... playPause: real 0m0.020s (~20ms)
• Confirms lag is not in keybind/shell/IPC wrapper.

Before patch (system musikcube build)

Command:
bash -lc 'for i in {1..8}; do echo "---- $i ----"; time playerctl -p musikcube.instance791874 play-pause; time playerctl -p musikcube.instance791874 status >/dev/null; done'

Observed:

• playerctl ... play-pause: typically ~2.5s (e.g. 2.500s, 2.560s, 2.603s)
• playerctl ... status: consistently ~0.5s (e.g. 0.500s, 0.509s, 0.551s)

Direct DBus reproduction (no playerctl wrapper)

bash -lc 'for i in {1..5}; do echo "== $i =="; time busctl --user call org.mpris.MediaPlayer2.musikcube.instance791874 /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player PlayPause; time busctl --user get-property org.mpris.MediaPlayer2.musikcube.instance791874 /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player PlaybackStatus >/dev/null; done'

Observed:

• PlayPause: 0.399s, 2.098s, 1.500s, 2.099s, 2.000s
• PlaybackStatus: mostly 0.500s (one 1.000s)
• Confirms lag originates in MPRIS handling, not playerctl.

After patch (local build)

• Built patched source successfully.
• Manual verification: play/pause control responsiveness became effectively immediate; prior multi-second delay no longer reproduced in normal use.
• UI responsiveness was preserved after adjusting the loop to avoid blocking while holding the mutex.