Set StartupWMClass in desktop file

Used by non-Plasma unixoid X-based shells to map windows to desktop files
e.g. in launcher UIs, which do not know about _KDE_NET_WM_DESKTOP_FILE and
instead expect (one of) the WM_CLASS entries to match the basename of the
desktop file by default. StartupWMClass overwrites that default.

kcachegrind/org.kde.kcachegrind.desktop

@@ -170,6 +170,7 @@ Comment[uk]=Візуалізація даних профілювання шви
 Comment[x-test]=xxVisualization of Performance Profiling Dataxx
 Comment[zh_CN]=性能测试数据的可视化表现
 Comment[zh_TW]=效能分析資料視覺化
+StartupWMClass=kcachegrind
 X-DBUS-StartupType=Multi
 X-DBUS-ServiceName=org.kde.kcachegrind
 Categories=Qt;KDE;Development;