🍒shell: make accent color optional, and enable accent color after slot connection

1. m_accentColor in DesktopView now becomes an optional value, so when the current accent color is transparent, ShellCorona::colorChanged signal can still be emitted without being blocked by accentColor == m_accentColor.
2. The ShellCorona::colorChanged signal may be emitted earlier than m_fakeColorRequestConn is established, so connect to the slot before enabling accent color from wallpaper.

STR:

1. Disable accent color from wallpaper
2. Switch to "Cold Ripple" wallpaper
3. Restart plasmashell.
4. Run qdbus-qt5 org.kde.plasmashell /PlasmaShell org.kde.PlasmaShell.color

Before this commit, the command will hang and quit with error. After this commit, the command will print "0".

BUG: 470280 FIXED-IN: 5.27.6

(cherry picked from commit 4c08bf02)