A3t

- Added ::clear method.
- Handles ::captureFirst with an empty tree.
CmdCalendar

- Converted from A3 to A3t.
Shell

- Converted from A3 to A3t.  Still doesn't work though.
CmdUndo

- Converted from A3 to A3t.
CmdBurndown

- Converted from A3 to A3t.
CmdCustom

- After injecting custom filter, re-parse.
Command

- Added filter expression debug diagnostics.
Code Cleanup

- Removed diagnostics from default command handling.
A3t

- Attributes in filter now properly expanded.
A3t

- Reordered recognition of PSEUDO, UDA and ATTRIBUTE, which is a better
  precedence.
A3t

- ::getFilterExpression expands /pattern/ to description ~ pattern
A3t

- Stubbed ::patchInfix to insert missing and implied logical operators.
A3t

- ::getFilterExpression expands +tag to tags _hastag_ tag,
  -tag to tags _notag_ tag.
A3t

- Stubbed attmod expansion, copied over from A3.
A3t

- Inserts implicit AND operators into filters that otherwise don't.
Filter

- New Filter object to control filtering, instead of commands/Command.cpp.
Eval

- Added support for compiled expressions that are compiled once and
  evaluated multiple times in different contexts.
CmdCount

- Converted from Command::filter to Filter::subset.
Filter

- Added domSource, and a context Task so that Eval can access attributes.
  Doesn't work.
Filter

- rc.debug=1 propagates to Eval::debug.
- Corrected comments.