Fix element booting, holder writes, db path names and invalid route expressions

[samuelpatro]

Four unrelated correctness bugs. Each has a test that fails on develop.

• ElementBase::__construct() never called parent::__construct(), so ::extend() callbacks and $implement behaviors were skipped for constructed elements but applied to unserialized ones. Extensions now boot after the config is applied, as they do in __wakeup(). Methods they add are now called instead of being stored as config by the fluent setter.
• ElementHolder::get() kept returning the first value it read, so a later $holder['key'] = 'new' was never seen. It now reads the config each time and still records touched elements.
• DbDatasource::pathToFileName() used str_replace(), which removed every occurrence of the directory name: pages/pages/about.htm became about.htm and pages/blog/pages.htm became blog/.htm. It now strips only the leading directory.
• A route segment with an invalid expression, such as :id|[unclosed, matched any value. The ErrorException Laravel raises for the preg_match warning was swallowed and the segment counted as valid. Router and Rule now treat it as a non-match.

[samuelpatro]

@daftspunk rechecked this against current develop and changed a few things:

• ElementBase now boots extensions after initDefaultValues() and useConfig(), so ::extend() callbacks see the configured element, the same as after __wakeup(). Constructing a FieldDefinition goes from 0.92 to 1.13 µs.
• Methods added by behaviors or extend() callbacks are now called, instead of being stored as config by the fluent __call() setter.
• DbDatasource::pathToFileName() trims a trailing slash from the directory. buildDirectoryQuery() already accepts select('pages/'), and the previous version returned pages/about.htm for it instead of about.htm.
• Dropped the ElementHolder::offsetUnset() override, so unset entries stay touched and Form still resets their widgets as before.
• Tests go through select() instead of calling pathToFileName() directly, and the two router tests are now one. Each fix has a test that fails without it.

Ready for another pass.

[daftspunk]

Thanks!