Port QRegExp::exactMatch() by using QRegularExpression::anchoredPattern(),
unless the pattern is already anchored by "^" and "$".

BacktraceLineGdb::parse():

• set QRegularExpression::DotMatchesEverythingOption, since gdb breaks some stack frame lines to mulitple lines for readability, e.g.: "#5 0x00007f50e99f776f in QWidget::testAttribute_helper (this=0x6e6440,\n attribute=Qt::WA_WState_Created) at kernel/qwidget.cpp:9081\n" this matches QRegExp behaviour where a "." char matches new lines by default.
• Use clustering parentheses (?:) so as not to capture groups that aren't needed, less bookkeeping for QRegularExpression/PCRE

Replace QRegularExpression with QString comparison, the latter is usually
faster.

All unit tests still pass.