feat: show all themes

[cathaysia]

还记得上次我留下的注释吗？

# At present, it seems that the function of finding themes is based
# on dirs, but .kvconfig. So some theme will no be recognized. This
# may be fixed next time

准确地来讲，kvantum 的主题是由文件夹中的 kvconfig 文件定义的，如果只是根据 /usr/share/Kvantum 中的文件夹来判断主题的话，就会导致一些主题的丢失。这点可以通过安装 fluent 主题确认，其所有的 dark 变体都会丢失。

现在，直接查找路径下的所有 kvconfig 文件，可以准确识别 kvantum 中的所有主题

[l0drex]

Looks very good! Can you please do these changes in the beta branch? I have lots of changes in there that should be available soon. To avoid possible merge conflicts, it would be nice to collect everything in there.

[l0drex]

Interesting, turns out I can do that too! Thanks again, I will merge this in version 3.2 to master