Rewrite book loading functionality

[pachpict]

Aim is to avoid hang-ups and unresponsiveness. Relates to and is intended to fix issues #192, #205, #206

@timburg1 has done a thorough investigation of book loading behaviour here: https://github.com/Bristol-Braille/canute-ui/wiki/Testing-report-by-TB-from-2019-07-24-to-2019-07-30

The report references book loading performance through-out, but here are the specific tests that came up with book loading problems: https://github.com/Bristol-Braille/canute-ui/wiki/Testing-report-by-TB-from-2019-07-24-to-2019-07-30#test-1-1 https://github.com/Bristol-Braille/canute-ui/wiki/Testing-report-by-TB-from-2019-07-24-to-2019-07-30#test-2 https://github.com/Bristol-Braille/canute-ui/wiki/Testing-report-by-TB-from-2019-07-24-to-2019-07-30#test-3 https://github.com/Bristol-Braille/canute-ui/wiki/Testing-report-by-TB-from-2019-07-24-to-2019-07-30#test-5 https://github.com/Bristol-Braille/canute-ui/wiki/Testing-report-by-TB-from-2019-07-24-to-2019-07-30#test-15 https://github.com/Bristol-Braille/canute-ui/wiki/Testing-report-by-TB-from-2019-07-24-to-2019-07-30#test-18

[zwbrbr]

Needs feedback.