Fixed|libdoomsday|Thinker: Crash at shutdown after zapping a thinker

Zapping should retain the malloc flag of a thinker so that it can
be later freed using the appropriate function.
Fixed|XG|libcommon: Misuse of dummy mobj in XG leading to a crash

p_xgsec was converted to C++ with some minimal cleanup. XG now allocates
thinkers using ThinkerT, and does not call memset on the dummy mobj
(which is a C++ instance).

IssueID #1855
Fixed|World|Plane: Accessing plane sound origins

The sound emitter property was being accessed as a pointer to ThinkerT
rather than a pointer to ddmobj_base_t.

IssueID #1853
Merge branch 'master' into gl2-models
Console|UI: Menu option for copying log file path to clipboard

IssueID #1772