chore(idea): add project icon and un-ignore it in .idea

Adds a pictogram as .idea/icon.svg, shown by IntelliJ IDEA in
the window title, tab and recent-projects list. Carves out an
exception in .gitignore so the icon is shared via git while the rest
of .idea/ (user-specific IDE state) stays untracked.

Author: spannm

.gitignore

@@ -9,7 +9,8 @@
 **/NOTES.txt
 *.iml
 .checkstyle
-.idea
+.idea/*
+!.idea/icon.svg
 .~lock*
 *.kate-swp