applets/systemtray: Don't unnecessarily append -symbolic to icon names

Reason for the change

41d8bb43 removed a check that the icon name ends with "-symbolic" in favor of a function call to iconLoader()->hasIcon(iconName + -symbolic). However this introduced a subtle bug: it will return true for icons that don't exist, if such icons have an acceptable fallback that does exist.

This is not the behavior we want; we only want it to check the icon theme to see if a symbolic version of the icon name exists if we know it does not already end with -symbolic.

Test plan

Launch KTorrent or KMail with their system tray icons enabled -> tray icons are symbolic, not colorful.

Bugs fixed

BUG: 500102
FIXED-IN: 6.3.2

@davidre