Allow multiple component selection

[CGrassin]

Allow to select multiple components at once. The behavior depends on the format:

• Formats with "structures" shall generate one structure per component, e.g. Python below. Formats: C enum, Python dict, HTML Table, MD Table.

  pinout_U1 = { pin1: .....}
  pinout_U2 = { pin1: .....}

• Formats without structures shall instead display an error message "Multiple component selected (). Select only one to generate an output with this format." Formats: VHDL, C #define.

Notes:

• TBC how to handle CSV/List formats.
• This evolution is useful for documentation format, especially on connectors (e.g. https://github.com/CGrassin/kicad-pinout-generator/issues/1).
• Major code refactoring is required to implement this.

[CGrassin]

Added with https://github.com/CGrassin/kicad-pinout-generator/commit/d455a5af842c6a3104bc8c5aae369a7f7afde91d

Still TBC on CSV and List.

[CGrassin]

Fixed remaining TBC with b8e69bea79f1838e93bdafaea1ab5e989231c94f : for both CSV and List, a new column with the component reference is added when there is a multiple selection. Note: the current implementation means that the output format is different when in multiple selection compared to single selection.