Optimize keyword search

This changeset has a bunch of performance fixes to reduce the overhead of interactive search. In particular it:

• Cuts out hundreds of thousands of strlen() calls per keyword
• Cuts out hundreds of thousands of memory allocations and ucs4 to utf8 conversions
• Cuts out hundreds of thousands of calls to g_strstr_len() through it's slow path to strcmp() which can be hoisted/optimized by the compiler
• Avoids tens of thousands of calls to g_task_return_error_if_cancelled() now that search is generally fast enough to not warrant the function call overhead
• Inlines hex conversion for comparison avoiding calls to g_strtoul and strspn hundreds of thousands of times.
• Fixes a memory leak of significant amounts utf8 data
• Ensures that some conversion functions always leave a \0 terminated string
• Fixes a potential leak in some public API
• Reduces GNOME Shell search provider to max 20 results as only 5 are shown

Fixes #130 (closed)