fix(linux): Collect consecutive backspaces

When using Wayland `im-wayland.so` (which is part of libgtk) gets
loaded into the client app instead of `im-ibus.so`. This means that
our ordered output doesn't work with Wayland.

This change works around a problem with keyboards that use multiple
consecutive backspace actions. Because method calls get processed
asynchronously some backspaces get lost. In clients that support
surrounding text we can count the consecutive backspaces and then
do one call to delete surrounding text with the appropriate number
of characters.

This can be tested with "Khmer Angkor" keyboard by typing `xEjmr`,
or with "Vedic Sanskrit Devanagari Phonetic (ITRANS)" keyboard by
typing `shrI`.

Part of #4273.