Fix match count in Corpus Viewer widget

[leskovecg]

Issue

Fixes #1120

Description of changes

Added a new column "Match Count" to the Corpus Viewer widget that shows the number of regex matches per document when a filter is applied.

• Counts are computed using the same regular expression used in filtering.
• The counts are displayed dynamically and updated after filtering.
• Column is also added as a meta attribute when outputting the annotated corpus.
• A unit test is included to verify that match counts per document are calculated and stored correctly.

Includes

• Code changes
• Tests
• Documentation

[leskovecg]

Closing this PR in favor of a cleaner fork-based version: see #1129.