Asammdf

Fixes
* improve plot update performance

Fixes
* clear resources when closing files
* use MinimalListWidget in the search dialog
* improve Signal __init__ performance
* improve append in case the signals have no samples (0 cycles)
* avoid using the max builtin function to increase performance
* improve update_lines performance (GUI)

Fixes
* add itemsDeleted signal to MinimalListWidget
* improve fit and stack for curves containing NaN and inf
* add context menu to MinimalListWidget
* add possibility to load .cfg files in the file widget
* prevent max recursion error in the numeric window

Fixes
* add itemsDeleted signal to MinimalListWidget

Fixes
* fixes 267
* improve keyboard shortcuts handling for the file and plot widgets

Fixes
* use MinimalListWidget for the can database list
* don't load windows if they have no signals
* in Tabular window clear filters that are applied to the index column when toggling the time as date checkbox
* fix missing "found_id_count" error when performing CAN extraction from the GUI
* correctly set the compression argument when performing an export from the GUI
* remove the Search setting from the menu since the search field was removed
* improve alignment in the mdf info tab
* fix CAN extraction of Motorola signals

Standalone GUI builds
* [Linux](https://ci.appveyor.com/project/danielhrisca/asammdf/builds/28932495/job/2ja8ihnrgwncverd/artifacts)
* [Windows](https://ci.appveyor.com/project/danielhrisca/asammdf/builds/28932495/job/ycrenpbs73siw80g/artifacts)