tests: make test-ui-color use absolute_import

tests: make test-url use absolute_import

tests: make test-trusted use absolute_import

templater: add parsing and expansion rules to process "templatealias" section The debugtemplate command is updated to show expanded tree, but still the template engine doesn't support alias expansion. That's why the test says "parse error" for now.

templater: add function to parse whole string as template expression This will be a parser of template aliases, and it can also be used for processing quoted string templates in map files. That's why this function isn't defined in the upcoming _aliasrules class.

parser: factor out _trygetfunc() that extracts function name and arguments This provides a customization point for templater. In templater, there are two ways to call a unary function: func(x) and x|func. They are processed differently in templater due to historical reasons, but they should be handled in the same way while expanding aliases. In short, x|func should be processed as syntactic sugar for func(x). _funcnode and _getlist() are replaced by _trygetfunc().

parser: make _getalias() return (alias, pattern-args) pair This allows us to factor out a function that extracts a function (name, args) pair. See the next patch for why.