Merge #79: Embed monospaced font

67f26319a0ca7e34e3db17d9330133622a40de09 gui: Add monospaced font settings (Hennadii Stepanov)
22e0114d055b0ac6cdc47af2968b5dc742ba055a qt: Choose monospaced font in C++ code rather in `*.ui` file (Hennadii Stepanov)
623de12d040af89ff7c25de6eb0a19c67179dc5f qt: Make GUIUtil::fixedPitchFont aware of embedded font (Hennadii Stepanov)
89e421918ee8b9c8439317f747e5c37f0733d94b gui: Add Roboto Mono font (Hennadii Stepanov)

Pull request description:

  Qt does not guarantee that the actual applied font matches to the requested one.
  It was noted (https://github.com/bitcoin/bitcoin/pull/16432#issuecomment-514486077):
  > the monospace font looks a bit weird no macOS

  ... because it is _not_ monospaced.
  Also some discrepancies I've noted on Windows while testing Qt 5.15 ([#19716](https://github.com/bitcoin/bitcoin/pull/19716)).

  Of course, we could check the actual font with `QFontInfo`, and try to choose another font.
  But this PR suggests to just embed a monospaced font, and get the GUI look (partially) independent from a platform.

  [Roboto Mono](https://fonts.google.com/specimen/Roboto+Mono) was chosen after discussion with Bitcoin Design community, and due to its [Apache License, Version 2.0](https://fonts.google.com/specimen/Roboto+Mono#license).

  Changes are scoped to the Overview page only.

  ---

  Screenshots on macOS 10.15.6 (images are simulated by code patching):

  - master (ca30d34cf94b7797272ef1920ca4b48716e7f999)
  ![Screenshot from 2020-09-03 14-10-03](https://user-images.githubusercontent.com/32963518/92107902-30357d80-edef-11ea-8a4f-b4c758eebf66.png)

  - this PR (3fdd5b6bd17a679d6e3876682266092159c52d59)
  ![Screenshot from 2020-09-03 15-41-36](https://user-images.githubusercontent.com/32963518/92116277-4b5aba00-edfc-11ea-8cb9-22fc44460bfb.png)

  ---

  More screenshots added after https://github.com/bitcoin-core/gui/pull/79#issuecomment-782909149:

  - Linux Mint 20.1 + Cinnamon DE

  ![DeepinScreenshot_select-area_20210221205410](https://user-images.githubusercontent.com/32963518/108635739-b327be80-7489-11eb-8851-ac89f61199ee.png)

  - Windows 10 (with depends)

  ![DeepinScreenshot_select-area_20210221205056](https://user-images.githubusercontent.com/32963518/108635741-b6bb4580-7489-11eb-8b6b-66be5551eb8c.png)

  - macOS Big Sur (with depends)

  ![DeepinScreenshot_select-area_20210221202917](https://user-images.githubusercontent.com/32963518/108635746-bd49bd00-7489-11eb-8cd2-cf4bb2273a6d.png)

ACKs for top commit:
  laanwj:
    Tested ACK 67f26319a0ca7e34e3db17d9330133622a40de09

Tree-SHA512: a59775570b8ce314669ede50a0b69f53e8a47a41e7eea428835013240f0ce9afcff6e4c258895455b56806417ed877e5b7a9522f1904e95a5f435db8ccf6078c

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